Inadequate ventilation may be one cause of hazards during procedural sedation. Apnea, or lack of respiration, can be caused by the drugs that the patient receives through the intravenous port during sedation. Central apnea occurs when the patient makes no effort or little effort to breathe and is often caused by giving too much opioid agent to the patient. Obstructive apnea occurs when the airway tissues are so relaxed that they block gas flow, thereby reducing the volume of air that the patient receives. Obstructive apnea during sedation is often caused by giving too much sedative agent to the patient. Respiration is usually monitored during sedation using a pulse oximeter that measures the oxygen saturation in the patient's arterial blood. Respiration rate is usually monitored during sedation using transthoracic bioimpedance or capnometry. Transthoracic bioimpedance measures the change in electrical impedance across the chest using the EKG electrodes, and is often unreliable. Additionally, transthoracic bioimpedance cannot detect airway obstruction since the patient's chest still moves during an attempt to force gas through an obstructed airway.
Capnometry measures the concentration of carbon dioxide in the exhaled air and calculates a breath rate by measuring the time between excursions in the carbon dioxide signal. Capnometry is expensive and is known in the field to detect breath attempts as true breaths. Because capnometry does not measure the total volume of exhaled carbon dioxide, only a small volume of exhaled carbon dioxide is needed to cause a detection of a true breath. An additional limitation of capnometry is that the signal is degraded when high oxygen flow is given. This is a challenge because when a patient is hypoxic, the oxygen flow is often increased; however increased oxygen flow dilutes the exhaled carbon dioxide, thereby diminishing the clinician's ability to monitor respiration in the patients that have a great need to be monitored.
Additionally, there is a clinical controversy regarding the use of oxygen during sedation. While some believe oxygen is beneficial, others have stated that addition of oxygen is detrimental because it allows use of excess opioid and sedatives and that added oxygen only delays detection of inadequate respiration by the pulse oximeter.
While oxygen is delivered clinically at fixed flow rate that runs continuously, most of the oxygen delivered is not actually inhaled by the patient. The majority of the supplemental oxygen is wasted as the patient is breathing out (exhaling). Furthermore, even when the patient is inhaling, there is only a brief time when the patient is inhaling with sufficient rate to take in all of the oxygen that is being delivered through the nasal cannula or mask. It may be helpful clinically to calculate the fraction of the supplemental oxygen that actually enters the patient's airways.
Consequently, there is a long felt need for a technology that could more accurately detect a patient's breathing, especially in patients receiving opioids and sedatives. This technology would desirably further differentiate between a true apnea and mere displacement of an oxygen delivery device. It may also be desirable to be able to determine how much of the oxygen delivered to the patient is being inhaled by the patient.